Welcome.
The main problem with your "group" (i.e. duo) is that you don't have a tank who can hog incoming damage from enemies. Both rogue and ranger have similar strengths and weaknesses as they're both dex-based classes. That said, "classes" in DOS are just templates you can alter during the game with little restrictions (attributes being the most prominent). So, you could give your "mundane" rogue or ranger some ranks in magic skills or toughen them up with some physical attributes and/or Man-At-Arms skills. The higher skills will be unavaiblable because they usually demand specialisation in a particular attribute. Since rogue and ranger are dex-based classes, it does suggest itself even more to give them some skills of one another. Mages can summon a variety of creatures, some of which can substitute as tanks or at least for distraction – if only for their limited lifetime.
DOS is all about freedom and experimentation, so don't hesitate to try unusual combinations. Don't give too much weight to others' preferences. Play what's fun to you. I would say that you can hardly "ruin" your characters on Normal difficulty. Some builds may require better tactics than others, and higher difficulties require more optimised characters. What difficulty are you planning to play on?
