There is another option...
Understand that I've never had truly small minis or truly large horses. BUT I do have shetlands that are 38" at the withers at maturity so were much smaller before and on up to 45" tall. Our full size horses have ranged from 13.3 hh on up to 16.1 hh.
Our stallions each runs with larger mares -
my 40" inch boy runs with a 1/2 arab pony that is 13.1 and and arab filly that is coming 4 that is 13.3 hh now. He herds them and he guards them - to include ocassionally running he fence during the spring breeding/foaling season. The 1/2 arab mare has produced two foals by him BUT not thru natural, pasture breeding. I've never seen him try to mount during heat cycles but he's a "shy" breeder during hand breeding and I may not be seeing it... I've hand bred him to other mares by pulling him out of his pasture and by introducing other mares into his paddock for up to 2 weeks at a time. He is great with the babies - and I've often left colts & fillies with him thru the summer of their yearling year with no problems (though I'd not recommend that for everyone!! and we DID watch/keep an eye on how they were all doing).
the 45" boy runs with a 13.3 hh arab, a 14.1 hh arab and a 16.1 NSH mare. He is/was my main herd sire for many years and ran w/ all the mares and foals without any problems - great with the weanlings, too, up until the spring/.summer of their yearling years - then he'd run both fillies and colts into the ground and savage them if they got to close to the "herd"... He has sired 1 foal each out of the two arab mares - live cover via stocks and a ramp to stand on for height... He can't be put with any geldings of any size - he will chase, harrass, attack and injure them - no matter their size. The one time I thought to try to pasture him with the 40" boy (during the winter & w/ the mares none in heat but in pastures away from them) - didn't work either! No different then full size stallions fighting - terrible to behold (welll....would have been beautiful in different circumstances) and costly in resulting vet bills...
There is a pasture between the two boys' pastures that houses shetland mares - some open/come into season regularly and some bred to each one (both paddock and hand bred).
When I first purchased the 40" stallion - both boys regularly ran their fence lines - showing off, marking and screaming. Both lost weight (but were in fantastic condition) & the 40" boy refused to eat that first summer (going down to skin and bones - terribly). I resorted to hand feeding (something I don't do, LOL) but it helped to get him back into shape... Putting the two mares in with him, worked. He does seem to do ok w/ geldings - tho currently don't have any with him... As he gets older, he may not work that way.
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So - can you put your mini guy in with the larger mare??