Yes you can use ladders.
But as you are an internal cleaner you might be better off buying a water fed pole set up. That way you will have the tools to market a total internal/external clean, which would not be possible with a ladder for windows above conservatories as an example.
If you just want the tools as an add on then the cheapest way is to buy a RO unit from here:
http://www.ro-man.com/and then buy a backpack and pole which will reach most windows. Maybe a 24ft xtel hybrid or something that closes up small. You could setup for around £600 initially.
Also, if you are an employer you would be better off providing your employees with the safest tools for them to do their job.
But you can still use a ladder if that is the way you want to go.
i know nothing about the rules of window cleaning
am not sure what you mean but having a minimum charge and pricing enough to want to turn up for the next couple of years are good rules to follow.
It makes a big difference coming home from a days work happy, rather than coming home thinking you want to drop a job cos you've underestimated the price.
ps ask for best places to buy backpack as prices do vary a little but are around £160 i think?? Couple of hundred for a RO and spend a couple of hundred on a decent pole to begin with. Rain water butt from BnQ at home to store pure water and 25 litre barrels or a tank in your van and you are good to go.