Oscars and plants generally don't mix since oscars like digging so much. Also, a 75 is just big enough for one Oscar, adding other preds will be pushing it in a tank that size, look to doing weekly to bi-weekly large water changes.
A 5 gallon would be on the small side for a 75 wet/dry, it would be a waste of time IMO.
A great rule for safety and media space is to enter LxWx4" into an aquarium calculator.
That would let the tank drain 3"+/- before the sump over flows.
http://boonedocks.net/fishtank/ftweb.php
So a 75 is 48x18x20
Entering 48x18x4 gives you about 15 gallons, you could get away with a 10, maybe, but I wouldn't try it personally. I use a 20 gallon sump on my 75 and I feel I should of gone a little bigger, but it handles a power outage great with 3" left in the sump before it overflows.
If you're going planted, I'd get another Fluval 405 or Rena XP3 (or two) and pack them full of Seachem's Matrix. I feel Rena is better then fluval and I'm sure I'm not alone here.
Check
www.bigalsonline.com or
www.drsfostersmith.com for great prices on filters.
If you can afford an Eheim, get one, if not a Rena is a very good canister.
If you're not doing planted, go with a 20-30 gallon wet/dry with 600-900GPH pump and your canister, or 600-900GPH wet/dry Emperor 400/AquaClear 110, and your canister.