1 - you can run as many graphics cards as you want, all different or all the same, whatever can physically fit
2 - you can mix AMD and Nvidia as long as they're running separate monitors
3 - why can you not run two screens from the R7-250 card? nearly every card made in the last 10 years can run 2 displays without a problem
4 - does your motherboard have onboard graphics? you can use that in conjunction with the R7 card
5 - ignore gamers who don't have a clue and always assume that you absolutely must SLI / CrossFire stuff, as if that's a law or some such BS
6 - experiment! it's the best way to learn. if something breaks, you have backups, and a restore point! you DO have backups, right? and you DO use restore points, right?