Eidolons don't generally change the way you play the game, they add to the options you have. As for as mains go, you probably won't see the need to change it unless you have something that makes that strong of an impact (let's say Belial). Event eidolons generally don't give so much you feel the need to change your main.
Sol... is well useful on almost all current content, as far as I know a lot of ppl run her even off-element because her skill set is so fucking good (awakened is just godly). You probably won't need to change your main element, but do focus on building your next best after your main is optimised, since element advantage will become more and more important as the game updates.
Refund gold meaning...? Buy gold? I'm not sure what you mean, but just in case, unspent gold expires in 1 year (exactly 365 days).