Strange issue. Just received two new Omnitiks using version 6.18, as I was installing them on an existing network using 5.26, I downgraded the Omnitiks, then downgraded the firmware to match. Now, when I plug them in, they won't boot up unless I hold the reset button for 1 second while applying power. Using this method they come up fine. However, if I unplug them and plug them back in the power light flashes, but the system doesn't boot.

Not sure what I should do here. Do I need to upgrade all equipment to 6.19?

Try netinstall.

Don't downgrade to a lower routeros version than what the boards originally had.
And there is no problem with an Omnitik on v6.18 to work with v5.26 clients, but consider just upgrading everything.

I realised that some omnitiks have 32 and some 64MB of ram. And I am not talking about UPA version. Those with 32mb I use are suffering of not enough memory running 6.x and those with 64mb are fine with 6.x.