1, First is the property yours or to be let, there are rules to adhere to.
If yours then by all means have the detection on the same system.
2,There are many pet tolerant sensors on the market, what is best? What pet (s) do you have. Will they be able to run up and down stairs? Too many factors to just say one make/product.
3,Personally I love Galaxy panels ( you might be able to buy one, but I doubt you could get a manual for it, trade only say Honeywell the makers).
Menvier and Scantronic are good ( supplied by Cooper Security). Then you have Premier by Texecom. All good solid panels for a reasonable amount of money.
As you can see there are many to choose from, what you have to decide is what do you want the system to do. Can it be expanded ( most can be) can it contact you in an activation? Do you want control Via Internet?
Remember the system should suit your needs, not you fit around its limitations as it used to be in the dark ages
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Get 3 local companies to offer a quotation, see what they advise. Remember we are here to help and if we do not get a sale from a quote, such is life. You never know you might take one up on their recommendations.