The first step to a successful adoptable is organisation. As you mentioned with slots, things can quickly get out of hand especially with interactive.

To prevent this, you can first create the main ordering forum - this is where you accept orders or place them in waiting. You won't do interactive here, this will only be for people who want to order.

Keep in mind, some adoptables have interactive as optional so you may or may not want seperate threads for dealing with these.

You could work it similarly to your own shop, let's say you have 6 interactive slots. Then unlimited reservations alongside a set amount of slots for the orders you're currently designing the adopt for. The forms would be different for both -

If a slot is available for an interactive -

They fill out an entire order

If slots aren't available -

They fill a pack name, pack number so you can list them as a next and PM them when you have the time

Then a seperate forum with no interactives can work similarly to this. If you'd prefer to keep them in the same forum, add to the reserve orders Interactive - Y/N. Then, create two seperate areas of reservations.

Ones who said Y to interactive go under one and move up when explores finish, you can just PM them to let them know their explore has begun in the explore thread. Ones who said N to interactive go in their own reservation and move up when you can complete their order - let's say you work on two at a time, one person has a finished explore you're making, you can then move one without an interactive up to fill the second slot and so on.

People then create a seperate forum for purchasing items, profiles of the people who have adopts - this allows you to keep track of who has which items, a 'kennel' if sorts to manage who has what pets, some have a breeding forum for using breeding items bought from the shop and applying it onto their adopts.

Sometimes items can affect their adopt, affect their explore, affect breeding or they can affect something you've implemented. E.g a training system, battling system, playing system

The biggest attraction to adopts are quality and quantity as with anything. Quality comes down to the adopt itself but quantity is about both expandability and flexibility of the adopts. Are they usable after they've been claimed, can they be used in an interactive, can they be used to earn a currency - develop abilities - evolve mutations or earn a title. Adopts always come down to basic factors but the same as art, there are limitless expanabilities.

Some people make a forum reserved for details and examples or their adopt species, describing distinctions of it, lore, abilities and extra details like mutations of the sort that would be of interest to people trying to purchase any.

Feel free to PM me for any more details you need - I tried to keep this as compact and simple as I could. I've commissioned from over 20 adoptable shops so I can assure you I know a thing or two about them - I'll be more than happy to answer any and all questions ;)

If you decide to do adopts with interactive, do let me know if you need any help at all. I used to help a friend with them and I adore doing them. I do have a lot of literate experience so I can assure high quality and for you I'd charge nothing at all. ;)

I'm glad you're taking interest in adopts. It takes a certain mind set and drive but I don't doubt you could handle it should you find your mojo for something like this.