Hello George, and now the not what you want to hear news. Your qualifications mean nothing in the alarm industry :innocent
Yes you can easily understand the theory of how things work and how big the power supply should be, but it doesn't work like that.
At the end of the day it comes down to cost
Intruder / CCTV / Access control
An intruder alarm company will already know that to install an intruder alarm with say 10 devices they know the panel can supply the amount of current required and they will put in this size of battery (Because that's what they always do)
Its similar with access control and CCTV
I have even met "alarm engineers" who have no idea how anything works, but they know if they put this wire here, that wire there and press these buttons in this order the alarm will work. In short there is no qualifications needed to work in the alarm industry, and so they pay accordingly. They even fit FCU's because that is what the bloke showed them what to do.
With CCTV, people can see the end result, so are often happy, access control is a thing most folk despise, intruder alarms are things that call you out at 3 in the morning are always going wrong.
Fire alarms
This is where a few people my not agree. Again forget anything about being an electrician, its all designed for you. Long gone are the days of mains fire alarms using pyro and gents sirens. The emphasis with fire alarms is its a life safety system and you need to be aware of the regulations and requirements of fire alarms. Fire alarms are a thing that only big places have, and they don't need one. (They also install emergency lights, and even these have their own regulations)
What to do?
Its your choice. There are some companies out there that do everything, but are they any good? do they really know the correct regulations for everything? I have seen folk with intruder alarms say they also have a fire alarm because that is what they believe, when all they have is a few smoke detectors connected to an intruder alarm. I would look at both, and decide which you like the sound of best (Either way you usually start at the bottom installing cables and devices) and then apply to a company that specialises in it, I wouldn't go to a company that one team does all, and as I have already said the money is often not that great, but then you will have only just finished your apprenticeship so will have no "real experience" / can live on "lower money"
In finishing
I really do wish you good luck, don't rush into making a decision, either can be a rewarding job.