Changing cassette, can I change ratios Not sure if I'm using the right terminology here. Last night I changed the chain on my cheap road bike (2008 Specialized Allez Triple), and then took it for a quick test drive but the chain was skipping under any load at all. Figured the cassette is well done as well.

The problem is it's very hard to find a 12-26T where I am for some reason, but there are stores with 11-26T in stock. Would this cause big issues? Anything I should check for before I buy and install?

Also, I am sure I've seen people do this, but right now everything is Shimano, but the only cassette's I can get are SRAM, shouldn't be an issue correct?
This would be completely fine. You only really have to worry when going to a much wider range. Smaller road derailleurs cannot deal well with big differences between the largest and smallest gear, but they can all handle a 15 tooth difference. Things only start to get compatible once you move beyond 11-28.  Shimano/SRAM are compatible for 8-9-10 speed cassettes. Things aren't compatible at the 11 speed level. According to this chart, Shimano and SRAM mountain Cassettes are compatible at 11 speed, but for the road there is a small difference in sprocket pitch.