which is grammatically fine to use?
The softness and sweetness of rice [was/were] perfect.
The right answer is either. You are talking about two properties (albeit properties of one substance), but you are considering them together. You can make them two separable properties or take them together.
A slightly different but identically-constructed sentence is
The swiftness and stench of the river make it impossible to cross.
Using the plural verb implies that each of the swiftness and the stench makes it impossible to cross the river. It wouldn't matter if the river flowed slowly (but still smelt) or was perfumed (but was still a torrent) because you couldn't cross it either way.
Using a singular verb ...
The swiftness and stench of the river makes it impossible to cross.
... implies that the properties go together and you might be able to cross if one of the properties were not present.
Consequently in your sentence,
The softness and sweetness of rice ____ perfect,
you can use either depending on what you want to convey. I would make that plural because the rice could be perfectly soft but not perfectly sweet or vice-versa, and you want to say that both apply.
The right answer is was. Even though there are multiple attributive nouns in front of it, rice is a singular noun.
Depending on the context you're using this in, you may also need a few more articles...
If you're referring to a specific dish of rice that someone made, and wanting to pay a compliment for the excellent cooking, then you need the following:
The softness and the sweetness of the rice was perfect.
If you're generally trying to comment that the characteristics of any rice at all are perfect... (are... because characteristic*s*), then you could comment that "the softness and sweetness of rice was perfect... to serve with fish for a first course." Or something along those lines.