Splicing is allowed even for completely robotic/non-corporeal factions which arguably don't even have genes to splice (e.g., Epistis, Riftborn, Remnant, and Pulsos). If you are playing as a custom faction, any 'Horatio' populations you encounter will not be considered your primary population, and thus, they will also be available for splicing.

Splicing does not copy in the collection bonuses, so you still have a valid reason for potentially wanting some minor pops to augment your major pop, since you only need 1 of a minor pop on any system to multiply output by, say, 15% or whatever the bonus is. If you want to maintain these minor factions in your empire, be careful not to over-eagerly use splicing the moment you have enough to do it instead, get at least one surplus population before splicing. These populations are immediately deleted, and can drive the number of that faction in your empire to 0, effectively making it extinct. Splicing requires the sacrifice of a certain number of population instances of the faction being spliced this starts at 2, with the next faction requiring 4, then 6, etc. You're allowed to combine multiple bonuses from multiple factions - in fact, this is the main strength of this faction affinity combining as many of these as possible to make your primary citizens much more productive than those of other factions. This bonus is a 'lesser' copy of whatever that population usually provides in its vanilla form often reduced by 1/3 or 1/2 however, it is provided in addition to the natural population trait of Horatios. It immediately adds a bonus based on the population you've chosen to splice to all instances of your major (i.e., Horatio, or some custom faction with this affinity) population. The 'Gene Hunter' faction affinity allows you to splice the genes of any non-Horatio population into your primary population. The Horatio are a relatively vanilla faction, which more or less plays entirely normally, except for one well-developed mechanic: Gene Splicing.

Horatio endless space 2