Do you know how far away the Ubiquiti equipment is from you?
The noise isn’t horrible, especially if that Ubiquiti equipment is pretty close. Then again, you are going to fight with noise from that Ubiquiti equipment. You might want to have a chat with your competitors about maybe sharing spectrum so everyone isn’t stepping on each other’s toes, especially because you will be using equipment that has incompatible GPS timing w/ theirs.
I have ran Mimosa and Ubiquiti equipment on the same tower, luckily for me I was in control of all the equipment so I could channel plan to my hearts desire. Mimosa equipment has much better filtering and doesn’t make as much noise outside of the channel it is using compared to Ubiquiti equipment. (At least that is my experience and thoughts)
In your current situation, I wouldn’t recommend using the A5-14 for all 40 clients. While it will perform alright for a few customers and you are benefiting from not having to shoot very far, you will run into issues with finding channels that are clear for all your clients. Probably your best option would be a couple A5c combined with horn antennas. While the cost per AP will be higher, you will lessen the noise your APs are hearing and maximize the signals you are getting to/from your customers.
I would also recommend using the C5x combined with really high gained antenna, that will allow you to use really low TX powers so you minimize the noise you make while still keeping ~-40 dB signal strengths.
With that combo, you will be limiting your noise issues a lot and still have really high signal strengths. That will minimize the number of issues you will run into. That said, you will have to be really good about making sure you align your C5xs well. Play around with the Mimosa Link Planner tool a bit and check to make sure you can get within 3 dB of what is predicted, match your chains and you will maximize your MCS numbers which will really make your life easy.