can i just clip the wires from the other plugs ...

Yes, this is perfectly feasible.
If the PSU has split rails (not likely though), this would actually be the preferable scheme than only using the existing drive plugs.

would i need to do any tweaking to make sure it doesn't fry my hard drives?

It's a regulated power supply, so the PSU shouldn't "fry your drives" unless you miswire it.
Of course you will have to connect the green “power on” wire to a ground/black wire.

also what wires would i need to use to connect the fans, these are fans that connect to the motherboard. i think it is red and black

Assuming that the fan requires +12VDC, then you need to use a yellow (+12VDC) wire for the + (probably a red wire to the fan) and a black (ground) wire for the - (the black wire to the fan).

The red wire from the PSU is +5VDC. Typically that voltage is too low to reliably start a 12V fan. I.E. have you noticed that a fan controller starts a fan at full speed and then slows the fan down, rather than try to bring a fan gradually up to speed?

i have 250 i was planning on using, but i have 300 and 400 watt.

Try summing the current (amperage) used by each drive for 5V and 12V.
Then compare those two sums to the capacities of each PSU.
You'll probably only need the 250W PSU for two drives.
Using a larger PSU than necessary will probaly cost more in electricity.
See Will a 500W SMPS consume more electricity than a 250W SMPS if they are powering computers with same specifications?

can i just clip the wires from the other plugs ...

Yes, this is perfectly feasible.
If the PSU has split rails (not likely though), this would actually be the preferable scheme than only using the existing drive plugs.

would i need to do any tweaking to make sure it doesn't fry my hard drives?

It's a regulated power supply, so the PSU shouldn't "fry your drives" unless you miswire it.
Of course you will have to connect the green “power on” wire to a ground/black wire.

also what wires would i need to use to connect the fans, these are fans that connect to the motherboard. i think it is red and black

Assuming that the fan requires +12VDC, then you need to use a yellow (+12VDC) wire for the + (probably a red wire to the fan) and a black (ground) wire for the - (the black wire to the fan).

The red wire from the PSU is +5VDC. Typically that voltage is too low to reliably start a 12V fan. I.E. have you noticed that a fan controller starts a fan at full speed and then slows the fan down, rather than try to bring a fan gradually up to speed?

i have 250 i was planning on using, but i have 300 and 400 watt.

Try summing the current (amperage) used by each drive for 5V and 12V.
Then compare those two sums to the capacities of each PSU.
You'll probably only need the 250W PSU for two drives.
Using a larger PSU than necessary will probaly cost more in electricity.
See Will a 500W SMPS consume more electricity than a 250W SMPS if they are powering computers with same specifications?

can i just clip the wires from the other plugs ...

Yes, this is perfectly feasible.
If the PSU has split rails (not likely though), this would actually be the preferable scheme than only using the existing drive plugs.

would i need to do any tweaking to make sure it doesn't fry my hard drives?

It's a regulated power supply, so the PSU shouldn't ?fy your drives".
Of course you will have to connect the green “power on” wire to a ground/black wire.

also what wires would i need to use to connect the fans, these are fans that connect to the motherboard. i think it is red and black

Assuming that the fan requires +12VDC, then you need to use a yellow (+12VDC) wire for the + (probably a red wire to the fan) and a black (ground) wire for the - (the black wire to the fan).

The red wire from the PSU is +5VDC. Typically that voltage is too low to reliably start a 12V fan. I.E. have you noticed that a fan controller starts a fan at full speed and then slows the fan down, rather than try to bring a fan gradually up to speed?

can i just clip the wires from the other plugs ...

Yes, this is perfectly feasible.
If the PSU has split rails (not likely though), this would actually be the preferable scheme than only using the existing drive plugs.

would i need to do any tweaking to make sure it doesn't fry my hard drives?

It's a regulated power supply, so the PSU shouldn't "fry your drives" unless you miswire it.
Of course you will have to connect the green “power on” wire to a ground/black wire.

also what wires would i need to use to connect the fans, these are fans that connect to the motherboard. i think it is red and black

Assuming that the fan requires +12VDC, then you need to use a yellow (+12VDC) wire for the + (probably a red wire to the fan) and a black (ground) wire for the - (the black wire to the fan).

The red wire from the PSU is +5VDC. Typically that voltage is too low to reliably start a 12V fan. I.E. have you noticed that a fan controller starts a fan at full speed and then slows the fan down, rather than try to bring a fan gradually up to speed?

i have 250 i was planning on using, but i have 300 and 400 watt.

Try summing the current (amperage) used by each drive for 5V and 12V.
Then compare those two sums to the capacities of each PSU.
You'll probably only need the 250W PSU for two drives.
Using a larger PSU than necessary will probaly cost more in electricity.
See Will a 500W SMPS consume more electricity than a 250W SMPS if they are powering computers with same specifications?