Well the recommendation of software depends on the type of design work you are doing. If you are designing logos for print I would certainly suggest you use a vector program. Since you mention you can't obtain Adobe for personal reasons, I would suggest Inkscape as you have stated.
If you are doing primarily web designs such as sites, banners, email flyers than Gimp will serve a purpose and I don't see why expanding beyond Gimp would be necessary unless you just want to learn other design software.
Now preference wise, I try to only use a vector program for certain tasks. I say this because I am allowed to take a vector logo and use it for print work or I can import it directly into my site designs. I have seen and started using Illustrator for even web design but it is a learning curve and pixel accuracy is some what difficult over custom guide shortcuts for Photoshop.
If you are interested in comments on the alternative solutions to Adobe you can head over to alternativeto.