Firstly, welcome to ribnet!
I'm not sure if you have taken the prop off your merc & put it on the Force 75 or whether the 15" one came with the Force engine, so I'll keep this at a high-ish level.
I assume you have a rev. counter, what you want to do is pitch the prop (excuse the pun) so that the engine is at it's max RPM at wide open throttle. I have no idea what your boat is, so can't guess at the weight etc the engine is trying to push along.
Your "bigger pitch = faster" theory is correct up to a point, that point being when the engine runs out of grunt to turn the propellor. The engine torque is fighting against two main things - the force to push the boat along and the drag to turn the prop through the water. This means if you have a light boat, you can put a smaller diameter propellor on (a lighter load means the prop doesn't need the bigger blades to "grip" the water) but because you have reduced the rotational drag, you can up -pitch and get a bit more forward motion.
It's all a bit of a balancing act. I got a small diamteter prop with my old Yam. Pushed the rib along at a ridiculous speed... at nearly 1000 rpm over the max
and as long as I didn't turn the wheel or meet a wave, at which point it just "let go" and started cavitating. I upped the diameter of the prop by about 1/2" which dropped the revs to the reccommendsed max and meant the boat would turn in it's length without bating an eyelid. I did loose some top speed tho'.
Back you your 50 / 75, if you've swapped the prop across, there is a very high chance that you'll over - rev. the Force as you have 50% more power avaialble to turn it. (also be aware that some Force engines had a 10 pole alternator, so your tacho may not be telling the truth) I'd suggest take it out, open the throttle gently, and see where your revs end up at full open. From memory 1" of pitch will add or subtract 200 rpm, so for example if your engine is max rated at 5500rpm, and you open up & get 5700 (don't stay there too long!) then you can up the pitch by an inch. Likewise if it only makes 5300, go down an inch in pitch (which may seem like the wrong thing to do, but your engine will rev. faster) If you ghave any local friendly boaters a lot of the Merc / Mariner props will fit, so if you know people who might let you borrow a prop, the best thing to do is to go out & test.
Long answer for a short question! Hope it helps.......