Most guests would not mind being a guest on a new podcast. Imagine being one of the first episodes of Design Matters. Saying this, there are some guests who may only accept invitations from podcasts with a certain level of listenership. Don't be worried, we all start somewhere!
I would say, yes, let them! Many people now are going on podcast tours to promote books, courses, and events in a more personal way than tv and traditional interviews. One thing to consider, you may have a better conversation about what they're promoting if they share a free or discounted version with you ahead of time. Don't be afraid to ask!
That is entirely up to you and your guest. Some podcasts are 45 minutes, some extend to even 4 hours. Be sure to ask your guest about time and communicate ahead of time. Don't be afraid of a long podcast! Sometimes when you aren't stressed about time, you can relax and have a great conversation.
A few different ways: