A few questions

Hey recently discover reddit lurked a whole bunch and now found out you a have a great dark souls community. Anyway here's my questions.

If you don't level a character can you invade any level? I know it's possible to invade any level as a pyromancer without levelling, just wondering could a warrior or bandit be able to as well?

Also would anyone have a fast roll solaire build on hand please?

And last question what's the best level to be able to get summoned in multiple areas?

Sorry if these questions have been already asked etc. :)