M And M Tiling

With a border, even a thin one, two separate tile sizes can work, but there are still things to watch out for. this depends on how many tiles there are of the 25x40 type. for example, if there were 3 in a row, then you could safely use 2 60cm tiles on the top without it looking out of place.

in less ideal situations, it would make more sense to cut some off of the edge of the sidemost tiles, to keep an even and clean appearance for the centre, the part of the wall you would be looking at the most.

For limestone tiles one would need a larger notched trowel (20mm or larger), plus the 10mm tiles, and you could expect a rise in floor level by even as much as 20mm in total. if you chose to not remove the kitchen units, i expect that would be fine, but you will need to refit the skirting board to be in line with the new floor height anyway.

Appyling more mortar will not fix whatever underlying issue causes them to crack so soon. you need to reapply a new layer of mortar to the affected areas.