The "general" rule for Goldfish of any kind ..fancy or just "feeder" types (Comets, to be specific) ...is 10g per fish. Sooo.. yep, Nemo will be fine in that tank for some time ..given that he is the only inhabitant.
If the drops remove the chloramines and other utility company "treatments" ..then you have the right one. Personally, I prefer to use Seachem's Prime, but there are a few that work okay ..get the job done ..etc.
In a few months, you may want to upgrade your Nemo buddy to a 20g ..just to give him optimum space and better water quality. Also, upgrade the filter ..with a 10g, you should have a filter that is guaged for 20g or more water... and with a 20g, it should have a filter made for 40g or larger. For example, I have a pair of Comets in a 40g tank ..had them 10 or so years, now ...and their filter is guaged for a 75g tank. Basically, it turns more gallons more times per hour.. gph.. and that's what you want with Goldies.
Goldfish are messy.. eat a lot.. poo a lot ...and pretty much tear up any decorations ..live or fake plants, ornaments and so on. Wilbur and Dorothy have been destroying their tank for years ..frustrating as heck! ..but it makes them happy! :wink3: