Hi, can I use rss and diamonds combined in upgrading my base?

Yes you can, and you can use diamonds to buy rss at any time if needed. When upgrading buildings there is always an option on the left to instantly upgrade using diamonds. (assuming you don't have any requirements for other buildings or population to complete first). Using the upgrade option on the right will start the upgrade timer and use the required resources, if you don't have all the resources you will get the option to buy them with diamonds when you tap this.

When using the 'get more' option to get more rss from your inventory you will also have an option here to spend diamonds to gain stocks and you can do this from the items menu as well.