2 different stock licenses (10 + 750) - use of credits + stacking of unused credits

You are using a Teams or Enterprise setting, I suppose? You should discuss this with your sales representative. They can answer that question and probably offer some testing configuration.

I'm not sure whether you can have two subscriptions running simultaneously; however, you might consider engaging in the 30-day Free Stock trial subscription which allows 10 images to be licensed at no charge. There is also a Free Collection that allows learning up to ~3 images per day, though that collection only has ~1 million images. This would allow your team to assess whether you want a larger subscription. Be sure to cancel the trial within the first 30 days to avoid having it automatically convert to a paid annual subscription.