Welcome to the wonderful world of seahorses. I commend you on doing your homework before jumping in. It makes all the difference in my experience.
Mustangs or Zulus would be just fine for a beginning seahorse. Keep in mind the Zulus like their water a little cooler and should be kept in the range 72 to 75 optimally.
The whitei are also hearty and very nice. They have similar temp requirements so could be kept with either the Zulus or Mustangs. All 3 of these species could be kept together if you are interested in some diversity.
The Zulus are very cute little horses one of my all time favorites. I love the whitei and erectus as well.
35 to 40g is a very nice size. I would say you could keep 6 large horses, 8 medium and up to 12 smaller horses in a tank that size. Mustangs would be considered large, whitei medium and Zulus small.
As for tankmates….. the safest fish would be any of the following….
NON algae eating blennies
Clownfish – False Percula
The invertebrates suitable for seahorses include:
Soft and leather corals
Mushrooms with the exception of the Elephants Ear, which is capable of consuming a seahorse
Small and micro hermits including Scarlet Reef (Paguristes cadenati), Left Handed (Caleinus laevimanus), Blue Legged (Clibanarius tricolor) and Mexican (Clibanarius digueti)
Snails, including Asteras (Astera sp), Ceriths (Cerithium strercusmuscarum), Nerites (Nerita funiculate) and Nassarius snails (Nassarius variegates)
Sponges and gorgonia are typically difficult to keep and especially so in a in seahorse tank due to conflicting needs for water flow.