Not sure if this is how it works already but I think the best way to start would be to set elo by RGL div, so invite players don't have to do preliminary ranking games vs newcomers and ruin the fun (I'd imagine RGL thought this out tho)
Also I feel like changing elo solely off results in pugs might be a bad idea, when pugchamp came out people tried super hard playing vs b4nny and he had the lowest elo on the entire site for a period of time--imagine if having a low elo actually could de-rank you and prevent you from playing invite pugs for example