ThingSpeak can handle both int and float, and even strings (but it wont plot the strings). For example, the following two posts (copied from my api keys tab on my channel) both worked to display data on my channel plot. See the update a channel feed page for more options.
https://api.thingspeak.com/update?api_key=XXXXXXXXXXXXXXXX&field1=25
https://api.thingspeak.com/update?api_key=XXXXXXXXXXXXXXXX&field1=25.0