Printing resolution and image resizing advice.

Hey, first time poster here.

I imagine this is a pretty common question, but I'd love some advice from someone with first hand experience.

I'm currently looking to develop some templates where I can substitute different pictures of athletes easily - think sports actions posters.

Ideally, I'd like to offer prints in 8x10, 11x14, and 16x20. Here are my questions:

1) Can I create ONE template in a 16x20 document at ______ (240? 300?) ppi and then resize it to the other sizes when it's finished? Will it look the same at a smaller size?

2) IF I do need to create a different template for each size, is it then necessary to adjust the ppi for each? (i.e. 8x10 at ___ ppi, 11x14 at...etc)

Any tips on understanding this would be greatly appreciated.