Hello!

There's an idea for a TTRPG I have, and is mostly geared to solo play (it has multiple "modes" and scenarios), but it does include options where you can play with others if you want. So my question is... is a game that includes the ability to play with other people alright, so long as it can be played solo and that's the main thing? 

Yes, most solo games can be played with a group with minor tweaks. Designing a solo game is a bit harder, since you have to include basically an AI GM in the system. Those AI GM mechanics can usually become adventure building tools for a live person GM.