Mis primos se llevan bien con mis amigos.

Llevar by itself usually means to carry, to take, to wear, etc.

When you add se and bien:

• llevarse bien (con alguien) = to get along well (with someone)

So se llevan bien literally comes from something like “they carry themselves well”, but idiomatically it means “they get along well”.

If you said only llevan bien, it sounds incomplete or incorrect here, because without se it no longer has the meaning of “getting along (with someone).”

Se is a reflexive/reciprocal pronoun that refers back to the subject mis primos.

• Mis primos se llevan bien
  = My cousins get along well (with each other).

Here, se indicates a reciprocal action: they get along with each other.

So:

• Subject: mis primos → they
• Pronoun: se → themselves/each other
• Verb: llevan (3rd person plural) → they get along

With bien/mal, yes:

• llevarse bien (con alguien) = to get along well (with someone)
• llevarse mal (con alguien) = to get along badly / not get along

But llevarse can have other meanings in different contexts:

• llevarse algo = to take something away
  
  • Me llevé el libro. = I took the book (with me).
• llevarse X años = to differ in age by X years
  
  • Mis primos se llevan dos años. = My cousins are two years apart in age.

So llevarse is a flexible verb; llevarse bien/mal is just one of its uses.

Because the subject mis primos is plural.

• Mis primos se llevan bien… → primos = they → llevan (3rd person plural)
• If it were one cousin:
  
  • Mi primo se lleva bien con mis amigos. → primo = he → lleva (3rd person singular)

In Spanish, the verb must agree in number (singular/plural) with the subject.

You can say it, but the nuance changes slightly:

• Mis primos se llevan bien con mis amigos.
  Focus: my cousins get along well with my friends.
• Mis primos y mis amigos se llevan bien.
  Focus: both groups together get along with each other as one big group.

In everyday speech, people might use both and not worry about the tiny difference, but grammatically:

• First version: one clear subject (mis primos) getting along with another group.
• Second version: two subjects (mis primos y mis amigos) who mutually get along.

With llevarse bien/mal, Spanish normally uses con to introduce the person you get along with:

• llevarse bien con alguien = get along well with someone
• llevarse mal con alguien = get along badly with someone

Other prepositions (like a, de, en) do not work here.
So Mis primos se llevan bien con mis amigos is the standard, correct pattern.

Yes, you can say:

• Mis primos se llevan bien con ellos.

This is natural if ellos (they) has already been mentioned in the conversation and it’s clear who they are.

Difference:

• con mis amigos: explicit; we know exactly who you mean (your friends).
• con ellos: uses a pronoun; meaning depends on previous context (who they refers to).

Grammatically both are fine; the choice is about clarity and context.

No, they don’t have to be male. Spanish uses masculine plural for:

• All-male groups
• Mixed groups
• Groups of unknown gender

If everyone is female:

• Mis primas se llevan bien con mis amigas.
  
  • primas = female cousins
  • amigas = female friends

If one or more are male in a group, you use the masculine plural:

• Mis primos (mixed or all-male cousins)
• Mis amigos (mixed or all-male friends)

In normal, everyday Spanish, bien goes after the verb:

• Mis primos se llevan bien con mis amigos.

You can add words like muy:

• Mis primos se llevan muy bien con mis amigos.

Forms like Mis primos bien se llevan con mis amigos are not natural in standard spoken Spanish. Something like Bien se llevan mis primos con mis amigos might appear in poetry or very stylized language, but not in regular conversation. Stick with:

• se llevan bien (con...)

You have two very common options:

1. Negate bien:

   • Mis primos no se llevan bien con mis amigos.
     = My cousins don’t get along well with my friends.

2. Use mal instead of bien:

   • Mis primos se llevan mal con mis amigos.
     = My cousins get along badly with my friends.

Se llevan mal is usually a bit stronger than no se llevan bien.

Llevarse bien/mal is mainly used for people (or sometimes animals) and their relationships.

For things like colors, clothes, or objects that “go well together,” Spanish usually uses other verbs:

• Estos colores combinan bien. = These colors go well together.
• Ese pantalón va bien con esa camisa. = Those pants go well with that shirt.

Saying Los colores se llevan bien is not natural; use combinar or ir bien instead.

No. In Spanish you almost always need a determiner (like mis, tus, sus, los, unos) before plural count nouns in this kind of sentence.

Correct options would be:

• Mis primos se llevan bien con mis amigos.
• Los primos se llevan bien con los amigos.
• Mis primos se llevan bien con unos amigos míos. (= with some friends of mine)

But Primos se llevan bien con amigos (with no determiners) sounds ungrammatical and incomplete in standard Spanish.