Sounds like theyre trying to upsell you on equipment thats not needed.
An isolator is typically used to prevent parasitic draw or for running your system with the car turned off so it keeps your main battery charged. If youre using two batteries with similar resting voltage, which they should be since they are both lead batteries, an isolator is not necessary. Ive never heard of someone needing a regulator or charger for a second battery. Just wire the two batteries in parallel and send it. Like chevyboy said it would be best if they were both AGM but having one AGM and one flooded cell shouldnt cause any problems. Overcharging would only be a concern if your charge voltage is too high, the current draw will drop to near zero when the battery reaches a full charge.