Since you're working on what could converge into one product suite, have you considered offering a formal partnership instead of a strict hourly arrangement? They might be able to bring a steadier stream of work to you, and help provide you a more robust company platform to launch your own project (by sharing it).
You could always put an escape clause into your agreement that indemnifies any and all of your projects if the alliance fails to take off financially.
The only reason I suggest this is because you're actively considering doing the work for money, suggesting that your own project is underfunded (and less financially urgent than doing the work of others). They can obviously help you with this problem if they're offering to pay you.
A lot of deals are better than 100% of nothing.