You are correct. The direct and indirect pronouns can appear before the verb or attached to the infinitive. In the case of "necesito dárselo mañana", since we have an infinitive that we can attach these indirect and direct pronouns to it, and since we are "giving" "it" to "him", it sounds a lot better to attach it to "dar" rather than have it precede "necesito". The other way we could have said this is "se lo necesito dar mañana" but that just obfuscates things.
Also, indirect objects always precede direct objects no matter where they appear. Additionally, 'se' is used in place of 'le' or 'les' if followed by 'lo', 'la', 'los' or 'las'.
Now, if you give commands, you can also attach these pronouns to the command verb. For instance, "deselo mañana" or "deselo mañana a ella" translate to "give it to her tomorrow".