Mixing battery brands is generally not recommended and often isn't possible. Batteries are managed as a pack, with one "master" unit talking to the inverter over a comms cable and controlling the others. Different makes use different management systems and won't reliably talk to each other, so if you have, say, a Huawei battery and want more capacity, you'll usually need to add the same make again rather than bolt on a different brand.
When it comes to specs, the strongest advice is to look past the headline kWh figure. The discharge rate (continuous power output) is what determines whether the battery can actually run your high-draw appliances — a 5 kWh battery that only discharges at 2–2.5 kW won't cover a kettle, oven and dryer together. Other things worth paying for are a higher depth-of-discharge (how much of the capacity you can actually use), a higher cycle count (how many charge/discharge cycles before it degrades), and a solid 10-year warranty.
Watch out for sales pitches that justify a much higher price purely on "more cycles" or "supplies loads better" without numbers to back it up. Ask for the datasheet figures — usable capacity, max continuous discharge, cycle rating and depth-of-discharge — and compare like with like. That's how you avoid overpaying for marketing claims.