Added some guppies and platys to my 55 gallon.

Officially started the cycling process.

I'm using start right to declorinate it (good..? bad..?). I've been doing 50/50 water changes and I still can't seem to get the amonia levels down. Should I up it to 75%? Will the water changes stress the fish out?
I've been doing it once in the morning and once at night. But the amonia levels are still showing as "Harmful" and "Dangerous".

This is the same case for my 10gal Betta tank.

Sounds like it was too soon in your cycle to add fish. You also need to get a test kit that will give you a real level....not just dangerous. While dangerous is good to know that can be relative.

Is there a brand you recommend? How do I fix that I added the fish too soon? I can't just take back live fish or they'll flush them.

I would keep up with the water changes. 50/50 should be good. You don't actually want to change too much water because you will just be taking the cycle back a step each time.

I don't have a test kit recommendation off hand but I would get one with drops, not test strips.

I'll have to go grab some tonight. I'll keep up on my 50/50 wc's. I was also told to lower my water temp. Before I added my fish, the water was at a constant 79-80 degrees. It's now at 77 degrees. Is this ok? Sorry for asking so many questions, I've never cycled a tank before and I don't want to do anything that may hurt the fish or slow the process.

Ask all the questions you need to! That's how ya learn. The temp I am not sure on....others here have kept livebearers for a long time.

use prime it helps alot and if its cause you are over stocked use this site aqadvisor.com it will tell how much a water change should be and if you need more filteration

and for your fish cause you added them to early try stress zyme it helps alot its like 4 dollars a bottle

Also look at the decoration you have in your tank, sounds silly I know but it happened to me a few years ago.
Set up a 25G FW tank, cycled it for a few days and added a few guppies to get the cycle started, well the ammonia level ramped up as I would have expected. I admit I hadn't tested the water before I put them in as I just expected it to all work.
A couple of days later I checked and the ammonia level was horrendous!
I'm thinking holy c**p these guppies must be Sh*****ing like oscars. I did water change of 20%, couple of days later 30%, then a 50%.
The guppies died, so I did another water change, by now we are at about 2 weeks old.

The ammonia level still would increase and so did my level of bafflement.

In the tank in addition to some simple rocks, my g/f had put a large crystal quartz, about 5" x 5" x8", I hadn't said no, thinking that quartz crystal was effectively just a large lump of sand.
I took all the rock out and saw the ammonia level start to drop, and yes it was the crystal.
I've also heard of similar problems where someone was given gravel that has been used before, as in they bought a used tank with gravel.

I'm not sure how experienced you are, but here's the basic cycle of the water as I understand it.
Ammonia level rocketing tends to be normal in the cycling process, the ammonia then breaks down into Nitrites, the Nitrites breakdown into Nitrates, this tends to take about a week I've found, which is why I didn't even test my water during the first week. By changing your water frequently during the first week you would be effectively rebooting the whole cycle. The addition of fish effectively knocks the cycle back, as the waste from the fish again adds Ammonia, which becomes nitrites etc. This is where the bacteria in your filter is your friend, as it works on the waste breaking it down. Chlorine (the great enemy of us tank addicts) is great at killing these bacteria, which is why you always rinse your filter media in the bucket of water you have taken from the tank when doing a water change.
As for filtration itself, you can never have too much, is my opinion. I aim for twice the quoted capacity, so on my 2 55G tanks each has filtration levels the quoted equivalent level for a 100G+ tanks. One tank has 2 canister filters, the other a canister and a dual HOB.
This means that I can even change filter media in one filter one week, and have the other filter untouched. For myself it helps that I keep a record of what was changed and when. Then again I have 4 tanks, and my cichlids in my oldest tank are now 3-4 years old, so it's working for me.
Hope that helps a little and good luck.

As far as your Betta tank goes they don't have to be in a cycled tank. Just use the dechlorinater and they are good to go. We breeders do 100% water changes twice a week with no cycle time.