Colours & outcrosses
So, let's say we have breed A that can be outcrossed with breed B, but breed B can't be with A. Let's say breed B carries the pattern which is not accepted in breed A. Will the horse be rejected? Will I have to use unrealistic registry?
Malopolski can be outcrossed with TB. Let's say out TB makes overo baby (not good example because overo can't be homozygous) that is to be registered as malopolski. Will it be rejected?
Yes, unless you can find a registered Malopolski of that colour and get an appeal approved to add it.
Equus accepts a breed in only the colours which are proven to exist, not those which are possible to have by breeding to outcrosses which carry it.