How important is it that two 12v LiFePo4 batteries in parallel are same brand, size, and batch?
You can do this, but be aware that as a lithium-ion battery ages its internal resistance increases.
When you wire an old and new cell in parallel the new cell will initially supply more of the load current. This causes it to discharge faster until both cells share the load equally. When load current is reduced the old cell will charge the new cell for a while until they equalize again. When charging a similar effect occurs.
How much the old cell's resistance increases depends on how it is used. For lowest degradation it should be maintained at ~50% charge and the temperature should be kept below 25 °C. Charge/discharge cycles should be shallow, eg. 80% to 20% rather than 100% to 0%. If these rules are followed the resistance should not increase much in 6 months to 1 year.
So long as the cell resistances are similar, the current is not too high (ie. less than the charge/discharge ratings of a single cell), and the battery is not discharged too far (eg. no less than 3.5 V), this temporary imbalance should not be a problem. However it could be a problem if you intend to draw more current from the larger battery.
Mixing lipo and lithium ion batteries
But they may have different voltage curves and their specific energy may be in different voltages areas.
So lipos have energy in the higher voltage range and if you compare them to 18650 li-ions they may have their most energy in a lower area around 3,5-3,8v area
What effects does this have to parallell different performance cell?
I don't know more than I'm letting go of my setup of 18s 18650s and 2 s lipo and a 1slife4po. Not because of performance issues but I like to use a technology that is less risky. Also if noted I use a 1s of life4po which is kind of a safety cell as it will just "eat" over-voltage if charged to much for any reason. For my usage it's not needed but it adds a layer of security to my battery.
When paralleling two different battery packs of different types you may have one pack giving out 20% power and the other the rest, that is 80% when the whole pack is fully charged at 100%. However meanwhile and when it is being used this ratio may change as the packs charge each other to make the voltage even between the two paralleled packs. When you are reaching 0% SOC in totalt it may change as well and maybe so much that it's not useable. Or it will be useable. To start you need to follow and check as you go to as not anything bad is happening your batteries when they are being discharged or charged. Like that they don't get to hot to touch or similar.