Historically, Foreman does not play well with existing records and it assumes to have subnets under full control. It would ignore allocating IPs from specified “range” which is defined per individual subnet in the Foreman’s database tho, as long as you have all your records outside of that range you should be fine. For DNS there are no issues unless you try to allocate names which already exist (as Foreman performs DNS query to verify this - this can be turned off however I think).
Things are changing now, @grizzthedj is working on External IPAM provider which is now in Foreman core and the smart-proxy plugin is emerging at the mentioned URL and we hope to merge this into smart-proxy core to ship this by default. If you select this option, Foreman will use IPAM API to allocate new hosts, therefore you can have all your records on one heap and there should not be any conflicts. Currently only Infoblox and Netbox are implemented tho.