Thanks for the rundown on your new tank! Protein skimmers are a great addition to any seahorse tank regardless of what other filtration it may have. That’s a wonderful piece of equipment to have and I’m pleased to see that you have one installed on your seahorse tank. However, you should not be operating a protein skimmer until after your new aquarium has cycled and the biological filtration has been established. Otherwise, the beneficial work that a protein skimmer does by removing dissolved organics before they enter the nitrogen cycle can actually prolong the cycling process and reduce the population of beneficial nitrifying bacteria somewhat. So don’t operate your skimmer for the time being, Kris. Wait until your aquarium has completely cycled and you are ready to add your seahorses. Running your protein skimmer at that time will help increase the carrying capacity of your aquarium, whereas operating it now can reduce the carrying capacity in the short-term by limiting the number of beneficial nitrifying bacteria that build up within the biofilter to a degree. In other words, if the protein skimmer wasn’t removing dissolved organics before they entered the nitrogen cycle, there would be more ammonia to feed the nitrifying bacteria in your biofilter, which is what you want while the aquarium is cycling.
Okay, with a protein skimmer ready to go when the time is right to begin operating it, and two external filters to move water and provide mechanical and chemical filtration as needed, you should be in good shape providing at least one of those external filters also has biological filtration ability. Does either your canister filter or the other external filter include bio-balls, a sponge, or other porous media to encourage the growth of a dense population of aerobic bacteria, Kris? (Prefilters and filter floss don’t count, since they need to be removed and cleaned or replaced regularly, which disrupts or eliminates any beneficial bacteria that may have been growing on or within them.) If either of your external filters includes such bio-media, then you’re new tank should be in good shape in terms of biofiltration once it has completely cycled.
In that case, just be aware that the biofilter may need a week or two to adjust to the heavier bioload when you add the new seahorses, so feed them sparingly at first and keep an eye out for transient ammonia spikes after heavy feeding. Providing you acclimate the new arrivals properly and avoid overfeeding them, you should be good to go.
When it comes to adding live rock to your aquarium, that can be done at any time before or after your seahorses arrive providing you obtain live rock that has been precured. Live rock already contains its full complement of nitrifying and denitrifying bacteria, so as long as it’s been precured to avoid the die off of delicate sessile organisms that inhabit the rock, it can go directly into your aquarium and doesn’t need to be cycled per se. In essence, you can add instant biofiltration ability as well as denitrification to help control nitrite levels by adding precured live rock anytime you wish. However, for a seahorse tank, it’s always a good idea to run the live rock through the usual "debugging" procedures first in order to minimize the amount of bristleworms or unwanted hitchhikers that may be transferred into the aquarium along with the rock.
Best of luck with your new aquaria, Kris!