If you want 50 amp capacity, you'll want to use 6 gauge copper wire or 4 gauge aluminum wire. I put the GFCI in my main panel, and then ran to a sub panel with standard breakers. I think the 50 amp GFCI was around $70 or so.
Most breakers can physically fit, but if there is ever a problem or electrical fire, and they find out that you used a cross brand that wasn't on the approved list, you could be liable for any damages, and insurance may not pay out. You need to look at the manufacturer of the panel to look through their documentation to find the breakers that have been approved by them to be used in their panels.
For example, I have a siemens main panel and sub panel. For a while, I had some Square-D breakers in the main panel when I added some circuits to the garage, but these were not approved, so I recently changed them out to siemens breakers just to be consistent. Same with my sub panel - when I bought my 18 inch bandsaw, I needed to run a new 220v circuit, and home depot didn't have a simens, so I bought whatever they had, and used it for a while, but I also ordered the siemens breaker from amazon, and when it came in, I changed it out.
It's one of those things where you most likely won't have any problems since it physically fits and works just fine, but if anything does go wrong, you may be in trouble for what would end up being a small cost to get approved breakers.
I ran 6 gauge wire in my basement/garage, stapled to the beams. The sub panel I bought only has 8 spots, and I am now wishing I had bought one with 12 (I might still change it out, the sub panels themselves are fairly inexpensive, it would just be a bit tedious to change all the breakers and wiring out), because where I brew is next to where I do my woodworking, and it would be nice to have additional 220v circuits for more equipment. For brewing, I can have both 5500 Watt elements running at the same time, which is nice to be able to do.
I also added one of those cheap meters so I can see how much electricity I'm using through the sub panel (it shows voltage, amperage, wattage, and KWh used).