You can easily install a tach in your rio, even if it DOESN'T have one already. The great thing about the Rios is kia but in the wireing for all options, weather or not that paticluar one had it or not. Remove your gauge cluster, in the back where the 3 harnesses plug in look for the lead in the plug portion itself (on the cluster, not the harness) that does not have a lead going to it. I hope that makes sence. I also don't remember what harness it was on either LOL. Anyway, fund the wire in the harness that goes to that spot and splice into that for the green signal wire on your tach.
You can also go under the hood and get it from the diagnostics port by the fuse box. Looking at the port from the front of the car, it is the hole on the top, second in from the right side. Put your signal wire in there or splice into the wire from underneith.
As for the rims, 15's can be done. It is the width and ofset that you need to keep an eye on. I don't remember what the offset for the Rio is but I am thinking it is 40. I had 16x7 rims and had to slightly roll and flare my fenders so I wouldn't rub when I hit a bump. I also had my car lowered 2 inches tho. You can flare your fenders rather cheaply with a wooden baseball bat and a heat gun. Let me know if you want more info on that.
Matt