Hey! It all depends. Most likely you can add a decent amount of fish, but I would make sure the biological filtration is strong. I would add some ceramic bio media and add extra bacteria to make sure the tank is extra stable and can handle the additional fish (it overall makes everything better). Some examples of good media are marine pure gems, or seachem matrix bio media.
As far as fish:
You can add another clown if you want, BUT keep in mind aggression is always a possibility. I would add one that's noticeably smaller so they work things out better.
Other cool fish are firefish (only if you have a lid though)
A goby/pistol shrimp pair is always really cool! Just make sure your rockwork is stable and that your tank is suitable for them (coral frags on sand may get moved which is a big con).
Neon gobies are great, as well as clown gobies
Jawfish are also good as long as you take into consideration there substrate requirements
I think Gobies and Firefish are amazing fish though and you cannot go wrong with them. Most of them are very peaceful, and it can be cool getting a bunch of different types of gobies.
Also Blennies are amazing! Tailspot blennies are amazing characters and are one of the few suitable for a 25 gallon tank!
Personally I think having products like Dr Tims First Defense makes acclimating and adding fish better and helps them deal with the stress. Also food is important too! The probiotics and nutrition is LRS Frozen food is amazing and I highly recommend looking into the LRS frozen foods!
Let me know if you have more questions! I have a 30 and 10 gallon tank so I definitely have some good nano experience!
