Thanks for your question.
Tyk’s SaaS does not allow for customisation as it is hosted by Tyk in a multi-tenant solution. There are some customisations available if you look at our more Hybrid model and deploy your own Gateways locally.
Please can you share what kind of plugins or customisation you are looking at, and what you are trying to achieve? This will help us advise on the best deployment method.
Separately, we have many clients that start using our SaaS and then scale to On-Premise, but this is usually to do with the scale of deployments, traffic etc
Tamara & the Tyk team